Suppressing σ phase formation by rapid solidification to prevent embrittlement in a low-cost aged medium-entropy alloy

Abstract

Developing cost-effective high- and medium-entropy alloys (HEAs/MEAs) is essential to promote their broad industrial applications. A newly developed low-cost Fe45Cr25Ni10Mn15Al4C1 (at.%) MEA shows excellent compressive strength and ductility, but it is highly susceptible to drastic embrittlement when a substantial amount of σ phase is presented. The study herein explored the utilization of rapid solidification (RS) with high cooling rate to produce the MEA and compared it with hot-rolled (HR) sample. Two major phases, face-centered cubic (FCC) and body-centered cubic (BCC), exist in both RS and HR samples with high-density NiAl-rich B2 nano-precipitates inside BCC phase. RS sample contains two regions with different microstructural morphologies where central zone undergoes larger thermal gradient and cooling rate and exhibits acicular FCC phase in coarse BCC matrix, while ring zone possesses alternating dendritic structures. Uniform crystal orientations and decreased chemical segregation are additionally found in RS sample. The transformation from BCC to σ phase in RS sample is significantly suppressed, which avoids the embrittlement. This is attributed to several factors including the reduced grain boundaries, decreased dislocation stored energy, and lower Cr content in BCC phase of RS sample. These factors effectively prolong the incubation time of σ phase nucleation. In addition, diffusion is clearly involved as σ phase forms and stabilizes, identified by the huge variations in Ni and Cr contents of B2 nano-precipitates within σ phase. The present study may open a new pathway towards harnessing the formation of σ phase to develop strong and ductile HEAs/MEAs at low cost.